In this project we have been studying developmental adaptations of T. cruzi to the vertebrate host, and in particular, the molecular basis of adaptive changes occurring during the differentiation of epismastigotes (vector stage) to metacyclic trypomastigotes (infective stage). A. Characterization of a stage-specific factor which accelerates complement decay Culture supernatants from metacyclic and tissue culture trypomastigotes were shown to contain an activity which both accelerates the decay of and inhibits the formation of complement C3 convertases. This factor was characterized for its sensitivity to enzymes, its binding to lectins, and was shown to be contained within a group of molecules with Mr's ranging from 80-90 K. B. Analysis of stage-specific gene expression in metacyclic trypomastigotes In an effort to identify genes whose expression is turned on during the differentiation of infective stage parasites, mRNA was extracted from metacyclic trypomastigotes and epimastigotes and translated in vitro. Analysis by immunoprecipitation with stage- specific antisera revealed several gene products uniquely expressed in metacyclic trypomastigotes. A cDNA library was constructed from the metacyclic mRNA to clone examples of these stage-specific gene products. C. Studies on developmental control of intracellular survival in macrophages A model was established in which metacyclics developing in culture were shown to survive in mouse peritoneal macrophages while log phase epimastigotes were killed. Pre-treatment with mouse complement was shown to enhance parasite survival presumably by channelling uptake into "non-toxic" intracellular compartments.